Laminar flow reactors applications

Practical applications to laminar flow reactors are still mainly in the research literature. The first good treatment of a variable-viscosity reactor is... 

A laminar-flow reactor (LFR) is rarely used for kinetic studies, since it involves a flow pattern that is relatively difficult to attain experimentally. However, the model based on laminar flow, a type of tubular flow, may be useful in certain situations, both in the laboratory and on a large scale, in which flow approaches this extreme (at low Re). Such a situation would involve low fluid flow rate, small tube size, and high fluid viscosity, either separately or in combination, as, for example, in the extrusion of high-molecular-weight polymers. Nevertheless, we consider the general features of an LFR at this stage for comparison with features of the other models introduced above. We defer more detailed discussion, including applications of the material balance, to Chapter 16. 

Wilck M. and Stratmann F. (1997) A 2-D multicomponent modal aerosol model and its application to laminar flow reactors. J. Aerosol. Sci. 28, 959-972. 

Chapter 8 ignored axial diffusion, and this approach would predict reactor performance like a PFR so that conversions would be generally better than in a laminar flow reactor without diffusion. However, in microscale devices, axial diffusion becomes important and must be retained in the convective diffusions equations. The method of lines ceases to be a good solution technique, and the method of false transients is preferred. Application of the false-transient technique to PDFs, both convective diffusion equations and hydrodynamic equations, is an important topic of this chapter. 

In recent years, micro-structured reactors have attracted considerable attention for a variety of applications [61, 62]. Such micro devices are characterized by a laminar flow, and the very high surface-to-volume ratio they provide leads to increased mass and heat transfer, offering the potential for process intensification. 

As our first application, we consider the classical Taylor-Aris problem (Aris, 1956 Taylor, 1953) that illustrates dispersion due to transverse velocity gradients and molecular diffusion in laminar flow tubular reactors. In the traditional reaction engineering literature, dispersion effects are described by the axial dispersion model with Danckwerts boundary conditions (Froment and Bischoff, 1990 Levenspiel, 1999 Wen and Fan, 1975). Here, we show that the inconsistencies associated with the traditional parabolic form of the dispersion model can be removed by expressing the averaged model in a hyperbolic form. We also analyze the hyperbolic model and show that it has a much larger range of validity than the standard parabolic model. 

The fluidized bed reactor can also handle fast, complex reactions, with mixing and temperature control being especially good when stirring is provided. Unfortunately, the extent of back mixing is difficult to assess so that the residence time distribution of the reactants in the reactor is uncertain. In addition, only small catalyst particles can be used, and attrition, with the consequent breakdown and loss of catalyst, is a problem. Finally, a catalyst bed is adequately fluidized over only a comparatively narrow range of flow rates. More information about kinetic reactors can be found in reviews [33,34,50], Applications of the basket-type mixed reactor to liquid-solid systems are discussed by Suzuki and Kawazo [62] and by Teshima and Ohashi [63], and the development of a laminar flow, liquid-solid reactor by Schmalzer et al. [64], In the latter reactor the wall is coated with a catalyst layer. 

A billion cars and coimting, himdreds of millions of them with catalytic converters—this application is a landmark success of catalytic science and technology. Automobile catalytic converters are mostly monoliths— like ceramic honeycombs with porous catalyst layers on their inner wall surfaces. These monoliths are the most widely used structured reactors, the topic addressed by Moulijn, Kreutzer, Nijhuis, and Kapteijn. In contrast to the classical reactors containing discrete particles of catalyst and characterized by random and chaotic behavior, structured reactors are characterized by regular structures and predictable laminar flow. Structured reactors can be designed in full detail up to the local surroimdings of the... 

Data Assume that the reactors are long enough for the dispersion model to be applied and that laminar flow prevails at all points. The Beer Lambert law of light intensity, I, is applicable I/Io = exp(aCL), where a is absorptivity of the reactant gas mixture at a concentration, C, which absorbs the light of the CO2 laser and L is the path length. 

The use of monoliths as catalytic reactors focuses mainly on applications where low pressure drop is an important item. When compared to fixed beds, which seem a natural first choice for catalytic reactors, monoliths consist of straight channels in parallel with a rather small diameter, because of the requirement of a comparably large surface area. The resulting laminar flow, which is encountered under normal practical circumstances, does not show the kinetic energy losses that occur in fixed beds due to inertia forces at comparable fluid velocities. Despite the laminar flow, monolith reactors still may be approached as plug-flow reactors because of the considerable radial diffusion in the narrow channels [1]. 

Considerations along the above lines lead to analogous correlations for the Sherwood number for the description of mass transfer in a single channel. The application of the rather simple Nusselt and Sherwood number concept for monolith reactor modeling implies that the laminar flow through the channel can be approached as plug flow, but it is always limited to cases in which homogeneous gas-phase reactions are absent and catalytic reactions in the washcoat prevail. If not, a model description via distributed flow is necessary. 

An application of microfluidic reactors is the development of a membraneless fuel cell. Two streams, one containing a fuel such as methanol, the other an oxygen-saturated acid or alkaline stream, are merged without mixing. The laminar flow pattern in the narrow channel helps to maintain separate streams without the use of membrane separators. Opposite walls function as the electrodes and are doped with catalyst. Ion exchange, protons for the add system, takes place through the liquid-liquid interface. This is an example of a solid-liquid-liquid-solid multiphase reactor. ... 

J d) is plotted against d/d in Fig. 6-7 also shown are the curves for the two ideal reactors, taken from Fig. 6-5. The comparison brings out pertinent points about reactor behavior. Although the plug-flow reactor might be expected to be a better representation of the laminar case than the stirred-tank reactor, the RTD for the latter more closely follows the laminar-reactor curve for 6/6 from about 0.6 to 1.5. However, there is no possibility for 6 to be less than 0.5 in the laminar-flow case. Hence the stirred-tank form is not applicable at all in the low 6 region. At high 6 the three curves approach coincidence. Conversions for these reactors are compared in Sec. 6-7. 

Nu and Sh correlations developed for laminar flow [19] are often used to obtain transverse transport in both micro- and macroscale reactors [20]. Since the older correlations were developed using simplifying assumptions, they are not applicable for highly exothermic reacting flows and new correlations have been developed since [21, 22]. 

Laminar flow is important in engineering systems, such as flow in pipes, fuel cells, chemical reactors, microreactors, microfluidic, and nanofluidic devices just to name some applications. 

For comparison the case of a tubular reactor with laminar flow but without molecular diffusion is also shown in Figure 4.10.61, which is formally represented by Bo 6, see also Figure 4.10.58. Values of Bo that are less than this value are only reached for very low values of Re x Sc and low L/d values, whereby we have to keep in mind that the model and thus Eqs. (4.10.117b) and (4.10.114) are only applicable for L/dt > 0.04Re x Sc. 

---